Second electric field-induced alignment

The most traditional experimental determination of p is the electric field-induced second harmonic (EEISH) method, which requires the molecules to be aligned in solution by an electric field, by means of their static dipole moment (po). The EEISH signal is therefore proportional to po and to p <>c (projection of p on po), which is assumed to be equal to p in most cases. The bulk NLO properties are frequently evaluated as the efficiency of a powdered sample in second-harmonic generation (SHG), or as the d components of the x tensor. 

Second harmonic generation from surfaces has been shown to be a powerful probe of surface interactions [97], which can yield information about the structure and alignment of liquid crystal molecules on surfaces and at interfaces [98], The role of the interface is similar to that of the dc electric field in electric-field-induced SHG, that is, to provide a symmetry-breaking field the direction of which is defined by the surface normal. 

The centro-symmetry may, however, be broken by the application of an applied dc electric field [12] such symmetry can also be broken on a surface either as freely suspended film or by surface alignment modification technique that induces flexoelectric effect [36]. More recently, Sukhov and Timashev have shown that the centro-symmetry can also be broken optically [46]. The main obstacle in getting efficient harmonic generation then Is the phase matching of the fundamental and the second harmonic s wave vectors. 

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